This application relates to the art of compositions and, more particularly, to an organic-inorganic hybrid polymer composition and a method of making same. The invention is particularly applicable to compositions for applying optically clear protective thin films to the surfaces of plastic eyeglass lenses and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and that the composition can be used for other purposes as well as for coating other plastic substrate surfaces, such as transparent display cases, windows and crystals for covering faces of clocks, watches and other instruments.
Plastic materials commonly are used for ophthalmic lenses because they are lighter, easier to process and provide better impact resistance than glass. However, the surfaces of the plastic materials used in ophthalmic lenses are relatively soft and porous compared to glass, and this frequently results in reduced optical clarity due to abrasion and staining of the lens surface. This problem may be alleviated by coating the lens surfaces with an abrasion and stain resistant thin film that commonly is known as a hardcoat.
The most desirable materials for hardcoating lenses are inorganic oxides such as quartz, fused silica, glass, aluminum dioxide, titanium dioxide and other ceramics. Because thin films of these inorganic oxides are best applied in traditional processes that reach 1000xc2x0 C. or more, they cannot be used with lenses that are made of organic polymers which will decompose at such temperatures.
Inorganic oxides can be applied to organic polymers by such processes as chemical vapor deposition and the sol-gel process but it is difficult to achieve a good bond because of the inherent incompatibility between the inorganic coating and the organic substrate. The different coefficients of thermal expansion for the inorganic coating and the organic substrate tend to cause delamination. Inorganic films with sufficient thickness to adequately protect relatively soft plastic substrate surfaces may become brittle and are prone to crazing. Equipment for chemical vapor deposition also requires a large capital investment and, because the necessary high vacuum chamber is relatively small, the numbers and sizes of articles that can be processed is limited.
Polymer coating materials have been developed that provide better abrasion and stain resistance than the surfaces of the plastic materials that are used for ophthalmic lenses, and many of these coating materials include an inorganic component for enhancing the abrasion resistance of the coating. The abrasion resistant properties of these polymer coating materials increase with increasing crosslinking of the polymer molecules because the density and hardness of the protective film that is formed from the coating material increases. Most abrasion resistant polymer coatings are formed by either thermal or radiation curing. The thermal process involves a condensation reaction of reactive monomers or oligomers, while the radiation process involves free radical polymerization.
One measure of the degree of crosslinking, hardness, abrasion resistance and porosity of a coating is whether or not a protective film applied to a lens is tintable. Protective films formed from known polymer coating materials are tintable because the pores of the film are larger than the dye pigment molecules. In a known wet molecular adsorption tinting process, a coated lens is submerged in a dye bath or organic dye molecules and water maintained at 95-100xc2x0 C., and this elevated temperature expands the size of the pores in the protective film by different amounts depending on the degree of crosslinking in the coating polymer. In known protective films, the pores are large enough to be penetrated by the dye molecules which range in size between about 5-30 angstroms.
Highly crosslinked polymer coatings that are more abrasion resistant than the polymers used for ophthalmic lenses are disclosed in many U.S. patents, several of which are mentioned hereafter by way of example. U.S. Pat. No. 4,407,855 discloses a pentaerythritol-based polyacrylate or polymethacrylate composition. U.S. Pat. No. 4,954,591 discloses a tintable coating composition of polyfunctional acrylate, n-vinyl derivatives and ethylenically unsaturated copolymer. U.S. Pat. No. 5,246,728 discloses a composition of tri- and tetra-acrylates in butanol. U.S. Pat. No. 5,401,541 discloses a highly crosslinked acrylic copolymer that is derived from a multifunctional aliphatic acrylate monomer. U.S. Pat. No. 5,459,176 discloses a tintable composition of polyacryloylated alkane polyols.
Although polymer coating compositions of the type described in the above patents form protective films that are much harder than the surfaces of the polymeric ophthalmic lenses, the nature of the carbon-carbon and carbon-hydrogen bonds in the films is not changed. In addition, the improvement in abrasion resistance does not approach the abrasion resistance provided by protective films of inorganic oxides.
The hardness and abrasion resistance of organic polymer coatings is improved by mixing an inorganic oxide, such as silica, with the composition that is used to form the coating. These compositions may be thermally cured or may be cured by ultraviolet radiation depending on the polymer that is used. Film coatings produced with such compositions are clear provided the individual silica particles are well dispersed and smaller than the visible wavelengths of light.
The amount of silica that can be added to a coating material for ophthalmic lenses is limited by the requirements of avoiding agglomeration of silica particles and insuring good dispersion so that the silica particles will not be visible in the protective film. Polymer compositions that include colloidal silica are disclosed in many U.S. patents, several of which are mentioned hereafter by way of example. U.S. Pat. No. 4,499,217 discloses a dispersion of colloidal silica in a thermosetting polymer. U.S. Pat. Nos. 4,973,612, 5,075,348 and 5,188,900 disclose blends of multifunctional acrylates, unsaturated organic compounds and colloidal silica. U.S. Pat. No. 5,104,929 discloses a blend of colloidal silica in ethylenically unsaturated aliphatic and/or cycloaliphatic monomers. These compositions do not have chemical bonding between the silica and the polymer, and protective thin film coatings formed with such compositions tend to fail in a relatively short time.
Attempts to alleviate the problems inherent in the lack of a chemical bond between the colloidal silica and the polymer have included the addition of reactive silane compounds to the composition for modifying the surfaces of the colloidal silica particles or for reacting with same. Disclosures of such compositions may be found in many U.S. patents, several of which are mentioned hereafter by way of example. U.S. Pat. No. 4,348,462 discloses a radiation curable composition that includes colloidal silica, acryloxy or glycidoxy functional silanes, non-silyl acrylates, and catalytic amounts of ultraviolet light sensitive cationic and radical type photoinitiators. This composition is said to cure to a transparent hard coating with improved abrasion resistance. U.S. Pat. No. 3,986,997 discloses a composition that includes colloidal silica, hydroxylated organosiloxanes and a silanol condensation catalyst. U.S. Pat. No. 4,478,876 discloses a composition that includes a blend of acrylate monomer, colloidal silica and acryloxy functional silane. U.S. Pat. No. 5,426,131 discloses a composition that includes acrylic monomers, functionalized colloidal silica and acrylated urethane. U.S. Pat. No. 4,177,315 discloses the generation of silica within the composition by hydrolyzing tetraethyl orthosilicate and aging the composition followed by the addition of organic silanol compounds to modify the preformed silica. U.S. Pat. No. 4,211,823 discloses a composition that has one or more compounds selected from a group that includes an epoxy group, a silanol group and a siloxane group, plus silica particles and an aluminum chelate. U.S. Pat. Nos. 4,242,416 and 4,177,175 disclose a composition that includes an organothiol containing siloxane resin and colloidal silica. U.S. Pat. No. 4,355,135 discloses a composition that includes siloxane and colloidal silica, and that forms a protective thin film coating that is readily tintable by conventional dyes. U.S. Pat. No. 4,486,504 discloses a composition that includes hydrolysis products of acryloxy functional silanes and/or glycidoxy functional silanes, and colloidal silica. U.S. Pat. No. 5,102,695 discloses a composition that includes colloidal silica, polysiloxane and alkylated amine formaldehyde, and that forms a thin film coating that is highly tintable by conventional dyes.
In the compositions of the aforementioned U.S. patents, silica is used to impart inorganic properties to organic polymers for improving the hardness and abrasion resistance of the compositions. The silica usually is colloidal silica having a particle size of 1-100 xcexcm and is dispersed in water or solvent. As previously mentioned, the silica particles sometimes become visible in the protective thin film coatings formed from the compositions or otherwise interfere with the optical clarity of the lenses on which the coatings are applied.
Preformed colloidal silica particles are very porous and have a density that usually is in the range of 1.0-1.5 g/cm3 depending on the process used to form the particles. In comparison, fused silica has a density of 2.0-2.1 g/cm3. Because of this relatively low density and the accompanying high porosity of the preformed silica particles, thin film coatings formed with compositions that contain such particles are readily tintable by conventional dyes. The relatively porous preformed silica particles also are relatively fragile and do not significantly alter the relatively soft nature of the plastic matrix. By way of example, the structure of a thin film that is formed from a composition that includes a polymer and colloidal silica particles may be represented in a simplified form as fragile balls enveloped by relatively soft plastic resin.
For the above reasons, it would be desirable to have a film forming composition wherein a silica component is self-generated in situ within the solution during preparation of the composition, and is covalently bonded with an organic polymer component of the solution on a molecular level to provide an essentially single phase state that has no interface problems.
U.S. Pat. Nos. 4,173,490, 4,186,026 and 4,229,228 disclose compositions wherein tetraethyl orthosilicate, methyltrimethoxysilane and glycidoxypropyltrimethoxysilane are cohydrolized with water and acid, and wherein the amount of methyltrimethoxysilane is very high, such as about 50 weight percent. However, methyl is an inert organic group that dramatically reduces the possible degree of crosslinking bonds. Large amounts of methyl or phenyl groups commonly are included in these types of film forming compositions to reduce brittleness and minimize cracking at the sacrifice of film hardness. Decreased crosslinking reduces the density of a film formed by the composition so that it remains relatively porous and does not have optimum hardness. The absence of any curing compound also reduces the possible crosslinking reactions by silanol condensation and by ring opening polymerization of epoxy groups. Therefore, these compositions form thin films having a porosity such that the films also are readily tintable by conventional organic dyes. U.S. Pat. No. 4,547,397 also discloses a coating composition that includes tetraethyl orthosilicate, methacryloxytrimethoxy and/or vinyltriethoxysilane. Thin film coatings formed by this composition also do not provide optimum abrasion resistance to the surface.
It would be desirable to provide a composition that can be used to form protective thin film coatings having such a high density and low porosity that they cannot be tinted by the use of conventional dyes. Thus, the thin film coating would have a pore size at 95-100xc2x0 C. and below that is smaller than 5 angstroms so that the pores cannot be penetrated by conventional dye molecules in a wet molecular adsorption tinting process. Such coatings provide high optimum abrasion and stain resistance that are superior to the abrasion and stain resistance of known protective thin film coatings.
This application will refer to several standard tests that are used in the ophthalmic lens industry to quantify the abrasion resistance and adhesion of lens coatings, and a brief description of each test follows.
The Bayer test is one in a series of standard procedures for determining the abrasion resistance of coated lenses. An abrasive media is oscillated back and forth over the surface of a coated lens under specified conditions. The abrasive media is 500 g of Alumdum 1524, and a complete test process is 600 cycles at a speed of 150 cycles/min. The quantification of abrasion resistance is based on the optical measurement of haze gain due to scratches formed on the coated lens by the oscillating abrasive media. The quantification of abrasion resistance is based on a normalized difference of the haze gain measured on the coated test lens compared to the haze gain measured on an uncoated plano lens of CR-39 resin provided as a reference by the International Standards Organization, also known as the ISO. CR-39 is trademark of PPG Industries, Inc., for allyl diglycol carbonate monomer or diethylene glycol bis(allyl carbonate) resin.
The steel wool test is one in a series of standard procedures for determining the abrasion resistance of coated ophthalmic lenses. A standard #000 steel wool pad with 5 pounds of weight on top of it is oscillated across a coated lens at a speed of 100 cycles per minute for 200 cycles. The quantification of abrasion resistance is based on a visual comparison of the test lens to a standardized series of reference lenses. The quantification of abrasion resistance is based on a ratio of the haze gain measured on the coated lens compared to the haze gain measured on an ISO reference lens of uncoated plano CR-39.
This standard procedure is for evaluating the adhesion of a hardcoat or an antireflective coating on a lens. Using a cutting device such as a razor blade, six parallel cuts 1.5 mmxc2x10.5 mm apart and approximately 15 to 20 mm in length are made in the coating on the front or convex surface of the lens. Another six parallel cuts 1.5 mmxc2x10.5 mm apart are made in the coating perpendicular to the first set. This forms a cross-hatched pattern of squares over which tape is applied, such as 3M Scotch brand #600 and 8981. The tape then is pulled rapidly as close to an angle of 180 degrees as possible, and the percent adhesion is quantified by the amount of coating removed from the squares in the cross-hatched pattern. The 180 degree reference means that the tape is pulled back over itself in a direction that is nearly parallel to the lens surface.
This standard procedure evaluates the ability of a hardcoat or an antireflective coating to adhere to a lens and the susceptibility of the coating to crazing. A coated lens is subjected to ten cycles of thermal shock by submersing the coated lens for two minutes in a boiling salt water solution which comprises 3.5 liters of deionized water, 157.5 grams of sodium chloride, and 29.2 grams of sodium dihydrogen orthophosphate, followed by submersing the coated lens for one minute in water at 18-24xc2x0 C. Coating performance is quantified by whether or not coating layer detachment or complete delamination from the lens occurs, and by whether or not crazing of the coating occurs.
This standard procedure evaluates the ability of a hardcoat, an antireflective coating or a combination of both to adhere to a lens, and the susceptibility of the coating to crazing at an elevated temperature. A coated lens is subjected to six hours of thermal aging in an air circulating oven at 80xc2x0 C. and coating performance is quantified by whether or not crazing of the coating occurs.
An optically clear protective thin film for polymeric eyeglass lenses and other polymeric substrate surfaces has covalent chemical bonds between polymer and silica molecules.
The protective thin film preferably has a thickness that is between 1-7 xcexcm and most preferably between 1.5-5.0 xcexcm.
A protective film in accordance with the present application has a very high density and a very high hardness to provide excellent abrasion and stain resistance. The high density and hardness are achieved by a high degree of cross linking between organic molecules and inorganic silica.
The improved film has such a high density that it cannot be tinted with the use of conventional dyes that are used for tinting eyeglasses.
The improved film is formed from a coating solution that includes tetraalkyl orthosilicate, expoxyalkylalkoxy silanes, (meth)acryloxyalkylalkoxy silanes and solvents.
In a preferred arrangement, a polymerizable component of the coating solution is 20-50 weight percent of the entire solution. The tetraalkyl orthosilicate comprises 40-75 weight percent of the polymerizable component, the epoxyalkylalkoxy silanes comprises 20-45 weight percent of the polymerizable component, and the (meth)acryloxyalkylalkoxy silanes comprises 5-15 weight percent of the polymerizable component.
Between 20-80 weight percent of the solution is solvent, and 15 20-50 weight percent of the solvent is water.
From 0.1-0.5 weight percent of the solution is 2M HCl, and 0.5-2.0 weight percent of the solution is acetic acid to provide a solution pH that is 3-6.
A surfactant or wetting agent comprises 0.1-1.0 weight percent of the solution, and a catalyst or curing agent comprises 0.2-0.5 weight percent of the solution.
In one arrangement, the ratio of the amount by weight of expoxyalkylalkoxy silanes in the solution to the amount by weight of (meth)acryloxyalkylalkoxy silanes in the solution is between 15 to 1 and 0.2 to 1, and more preferably between 13 to 1 and 1 to 1.
In another arrangement, the molar ratio of water to the combined epoxyalkylalkoxy silanes and (meth)acryloxyalkylalkoxy silanes is between 1 to 4 and 3 to 1, and more preferably between 1 to 2 and 2 to 1.
The coating solution is prepared by mixing together tetraalkyl orthosilicate, epoxyalkylalkoxy silanes, (meth)acryloxyalkylalkoxy silanes, solvent, HCl and acetic acid, and stirring at room temperature to partially hydrolyze the silane groups until the solution appears to be clear by visual inspection. The solution then is heated to 60-70xc2x0 C. and stirred for 1-2 hours to completely hydrolyze all silane groups and form organic-inorganic hybrid oligmers.
The solution then is cooled back down to room temperature, followed by the addition of the surfactant and the catalyst, and stirring to completely dissolve the surfactant and catalyst.
The coating solution is applied to the surfaces of polymeric lenses which then are baked in an air circulating oven at a temperature of 90-120xc2x0 C. to completely polymerize the coating and form an optically clear protective film.
It is a principal object of the present invention to provide an improved coating solution for use in applying optically clear protective thin films to the surfaces of plastic eyeglass lenses and other polymeric substrates.
It is another object of the invention to provide an improved method of making a coating composition wherein silica is generated in situ within the coating solution mix during processing from the solution constituents.
It is still another object of the invention to provide a protective thin film in which organic and self-generated inorganic molecules are bonded together on a molecular level with covalent chemical bonds.
It is an additional object of the invention to provide a coating solution that does not contain preformed silica but that forms protective thin films that include self-generated silica molecules as part of a polymer hybrid.
It is a further object of the invention to provide an improved method for preparing a coating solution and for applying same to substrate surfaces in a protective thin film.
It also is an object of the invention to provide a protective base coat as a foundation or primer on plastic lens surfaces and other substrates beneath multilayer inorganic films deposited by chemical vapor deposition or sputtering methods.
It is a further object of the invention to provide a composition that cures faster on plastic surfaces.